1. Field of the Invention
The present invention relates to a reference voltage generator in a charging device.
2. Description of Related Art
Charging devices for charging batteries generally comprise an internal reference voltage generator for comparing the battery voltage with the reference voltage output by the reference voltage generating circuit, and controlling battery charging according to the detected voltage difference. Because the optimum charging voltage of the battery varies according to the temperature, the reference voltage generating circuit is built with output temperature characteristics matching the battery characteristics to achieve optimum control in charging devices that maintain a constant correlation between the ambient temperature and battery.
An example of a reference voltage generating circuit known in the prior art is shown in FIG. 1. This reference voltage generating circuit comprises resistors R.sub.51 and R.sub.52, n (where n is an integer) diodes D.sub.1.about.D.sub.n, and resistors R.sub.53 connected in series in this order between the constant-voltage power supply 1, which outputs a voltage with little temperature-dependent variation, and ground, and outputs the reference voltage V.sub.0 from between the ground and the output terminal 3 connected to the junction point between resistor R.sub.51 and resistor R.sub.52.
The reference voltage V.sub.0 can be expressed by Equation (1) where the voltage of the constant-voltage power supply 1 is V.sub.cc, the forward voltage of diodes D.sub.1.about.D.sub.n is V.sub.F, and the current flowing through resistors R.sub.51 and R.sub.52, n (where n is an integer) diodes D.sub.1.about.D.sub.n, and resistor R.sub.53 is I.sub.50. EQU V.sub.0 =V.sub.cc -I.sub.50.times.R.sub.51 EQU =(R.sub.52 +R.sub.53)V.sub.cc /(R.sub.51 +R.sub.52 +R.sub.53)+n R.sub.51 V.sub.F /(R.sub.51 +R.sub.52 +R.sub.53)[V] (1)
The temperature characteristic .differential.V.sub.0 /.differential.T of the reference voltage V.sub.0 to the absolute temperature T can be expressed by Equation (2) derived from Equation (1) if it is assumed that the voltage V.sub.cc of the constant-voltage power supply 1 has no temperature dependence. EQU .differential.V.sub.0 /.differential.T=n R.sub.51 /(R.sub.51 +R.sub.52 +R.sub.53).times..differential.V.sub.F /.differential.T[V/.degree. C.] (2)
From Equation (2), it is known that the temperature characteristic (.differential.V.sub.0 /.differential.T) of the reference voltage V.sub.0 is determined by the n diodes D.sub.1.about.D.sub.n, resistors R.sub.51, R.sub.52, and R.sub.53, and (.differential.V.sub.F /.differential.T). From Equation (2), it is therefore possible to obtain various combinations of n diodes D.sub.1.about.D.sub.n and resistors R.sub.51, R.sub.52, and R.sub.53 if voltage V.sub.cc is fixed and the value of the reference voltage V.sub.0 is determined, and the temperature characteristic (.differential.V.sub.0 /.differential.T) can be achieved for this number of combinations.
The number n of diodes D.sub.1.about.D.sub.n, however, is a discrete integer value. As a result, it is not possible to set any desired temperature characteristic (.differential.V.sub.0 /.differential.T) by means of the reference voltage generating circuit described above.